Synchronization of static inverters

ABSTRACT

A system for synchronizing the static inverter enables the phasing of the output from a static inverter in correspondence to an alternate power supply. The synchronized system compares the static inverter output with the alternate power source in a phase detector which outputs an error correcting signal which is used to adjust the static inverter master clock.

O United States Patent 1 1111 3,710,133 Ferraro [451 Jan. 9, 1973 541SYNCHRONIZATION 0F STATIC 3,614,461 10/1971 Speer ..307/64 INVERTERS3,348,060 10/1967 Jamieson ..307/66 3,229,111 l/l966 Schumacher et al......307/66 X [75] Ralph Femm' p'scataway' 3,564,275 2/1971 Eberle..307/29 [73] Assignee: Ward Leonard Electric Co., Inc.

Primary Examiner-William H. Beha, Jr. [221 Flled' Sept. 1970Att0rney-Bernard Malina [21] Appl. No.: 70,718

[57] ABSTRACT U.s. t 4 307/66, A system for synchronizing the staticinverter enables [5 th pha i g of the out ut from a static inverter incor- [58] Field of Search ..32l/4; 307/19, 23, 29, 64, respondence to analternate power Supp|y The 307/66 235; 318/6 synchronized systemcompares the static inverter output with the alternate power source in aphase detec- [56] References and tor which outputs an error correctingsignal which is UNITED STATES PATENTS used to adjust the static invertermaster clock.

- 2,594,801 4/1952 Rees ..321/l0 9 Claims, 3 Drawing FiguresSYNCIIRONIZATION OF STATIC INVERTERS BACKGROUND OF THE INVENTION It issometimes desirable to bring a static inverter into service or take oneout of service either for repair or to utilize an alternate powersource. In the process of introducing an alternate power source toreplace the static inverter or vice versa consideration must be given tothe frequencies of the two alternating currents. A difference infrequency between two power sources coupled together can result in asingle signal which has the appearance of being amplitude modulated dueto the power sources change phase relationship from in phase to out ofphase. The presence of this modulation can seriously affect theoperation of equipment such as a computer or certain types ofsynchronized motors. In order to eliminate the occurrence of adifference in the oscillating frequencies between the two power sources,a means of synchronization is necessary.

SUMMARY OF THE INVENTION This invention provides a system for comparingthe output of a static inverter to an alternate power source and therebyderive a correction signal which can be used to adjust the master clockof the static inverter.

It is the object of the present invention to provide a system forsynchronizing static inverters.

Another object of this invention is to provide a signal for adjustingthe master clock ofa static inverter.

A further object of this invention is to provide an economical systemfor synchronizing a static inverter with lumped constant components.

Other and further objects of this invention will be obvious upon theunderstanding of the illustrative embodiment about to be described orwill be indicated in the apended claims and various advantages notreferred to herein will occur to one skilled in the art upon employmentof the invention in practice.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of theinvention has been chosen for purposes of illustration and descriptionand it is shown in the accompanied drawing forming a part of thespecification wherein:

FIG. 1 is a block diagram of a typical static inverter I used in theprior art.

FIG. 2 is a block diagram of the preferred embodiment of the invention.

FIG. 3 is a circuit diagram for the phase shift network and phasedetector circuit of the preferred embodiment of the invention. I

FIG. 1 illustrates a block diagram of a typical static inverter used inthe prior art. The object of a static inverter is to convert a directcurrent (D.C.) source into an alternating current (A.C.) source fordriving equipment adapted for an A.C. source. A portion of the DC.source supplies the power needed to operate the static inverter whilethe remainder is converted into an A.C. output.

The operation of the static inverter is as follows:

Master clock input 2 drives flip-flop 3. Flip-flop 3 is connected to asquare wave output inverter 4 which essentially generates half of thesignal needed to obtain the desired A.C. output. Flip-flop 3 is alsocoupled to buffer flip-flop 5 which is used to drive the slave clock 6.Slave clock 6 drives flip-flop 7 which is coupled over to square waveoutput inverter 8. Square wave output inverter 8 essentially generatesthe other half of the desired A.C. output signal. Slave clock 6 iscoupled into from variable phasing circuit 9 which provides compensationfor the phase between the output of square wave output inverters 4 and8.

The outputs from square wave output inverters 4 and 8 are applied toreactor 10 and then to harmonic sup pressor 11 and then to filter 12.The output signal at terminals 13 are monitored by current limiter 14overload voltage limiter 15, voltage limit compensator l6, and voltagelimit compensator 17. The voltage limit compensators 16 and 17 samplethe peak and valley voltage limits of the output voltage and provideinformation to the variable phasing circuit 9 so that the generatedsquare waves from inverters 4 and 8 are symmetrical.

FIG. 2 illustrates the preferred embodiment of the invention. The staticinverter which is to be synchronized is inputted on line while thealternate or parallel power source inputs on line 101. Switches 20 and21 which are mechanically coupled are closed in order to bring about thesynchronization between the static inverter source in use and thealternate source to be used. Transformer 22 is used to step down thevoltage of the static. inverter source while transformer 23 is used tostep down the alternate source. Phase shift networks 24 and 25 are usedto correct the phase relationship between the static inverter source andalternate source respectively so as to make the two inputs compatiblewith phase detector 26. Phase detector 26 and amplifier 27 receivedirect current (D.C.) from power supplies 28 and 29 respectively. Powersupplies 28 and 29 receive their inputs from the alternate power source.

The inputs from phase shift networks 24 and 25 are compared in phasedetector 26. The output from phase detector 26 is a signal which isproportional to the phase difference between the two inputs. This outputsignal is applied amplifier 27 where it is amplified and then outputs toa voltage tunable master clock 30. Master clock is used to supply amaster clock 30 input to a static inverter such as the type illustratedin FIG. 1. In addition, the output from master clock 30 can be used fora three phase static inverter. The output from master clock 30 which isapplied to flip-flop 3 of the static inverter shown in FIG. 1, changesthe frequency of the outputs of inverters 4 and 8, which, in turn,produces a change in frequency at transformer 22. The sense of thischange in frequency at transformer 22, is such as to minimize theamplitude of the output of phase-difference detector 26. Hence, thefrequencies between the static inverter and the alternate power sourcewill become synchronized.

FIG. 3 is a circuit diagram for phase shift networks 24 and 25 and phasedetector 26. Phase shift networks 24 and 25 areessentially similar.Resistors 40, 41, 42 and 43 which are across the output of transformer23 form a voltage divider network. Similarly, resistors 46, 47, 48 and49 form a voltage divider for the output through transformer 22.Resistors 41, 42, 47 and 48 are adjustable in order to balance theoutputs from transformers 23 and 22.

Resistor 44 and capacitor 45 form the phase shift 1 capacitor 51 formthe phase shift control for phase shift network 24. The output fromphase shift network 25 is coupled through resistor 52 to operationalamplifier 57. Similarly, phase shift network 24 is coupled tooperational amplifier 58.

Functionally, operational amplifier 57 applies switching pulses to thebase of a transistor 61 while operational amplifier 58 acts as adifferential amplifier receiving input signals from both the staticinverter source and the alternate power source. Resistor 52 andcapacitor 54 produce a phase lag of 90 with respect to the input signalfrom the alternate power source. Resistors 55 and 67 are used toincrease the input impedance to operational amplifier 58. The feedbackresistor 68 with resistor 67 determines the gain of operationalamplifier 58. Resistor 56 provides a bias and resistors 59 and 60 arebalance adjustments to set operational amplifiers 57 and 58 for a zerooutput. The supply voltages V and V come from power supply 28.

Diode 62 is used to block positive going output from operationalamplifier 57 to protect transistor 61 from excessive reverse voltages.Resistor 63 is the load resistor for transistor 61. The networkconsisting of resistor 64 and capacitors 65 and 66 form an integrator.The output voltage is the integral of the voltage present at the outputof operational amplifier 58 during the period of each cycle thattransistor 61 is switched on. The output voltage will zero, i.e., beingphase of the phase of the swing around a positive voltage, the outputvoltage being greater when the phase of the static inverter source lagsthe phase of the alternate power source and being less if the staticinverter source leads. The output is applied to amplifier 27. Becauseofa translation in voltage between power supplies 28 and 29, the output ofamplifier 27 swings plus and minus around zero of the control voltagefor master clock 30.

Amplifier 27 is of a conventional design is operative to amplify andmatch the output impedance of phase detector 26 to the input impedancemaster clock 30. Master clock 30 is a conventional voltage tunableoscillator whose output frequency is a function of its D.C. inputvoltage.

As various changes may be made in the form of construction andarrangement of the parts herein without departing from the spirit andscope of the invention and without sacrificing any of its advantages itis to be understood that all matter herein is to be interpreted asillustrative and not in a limiting sense.

Having thus described my invention, 1 claim:

1. An apparatus for synchronizing the output frequency of a staticinverter with the output frequency of an alternate power sourcecomprising phase detector means for deriving a correction voltagecorresponding to the phase difference between said static inverteroutput and said alternate power source signal output, means for couplinga signal from said static inverter output and a signal from saidalternate power source output to said phase detector means, means forsampling said correction voltage to produce a correction voltage signal,means for coupling said integrated sample correction voltage to avoltage tunable master clock and means for coupling said voltage tunablemaster clock to said static inverter for controlling the outputfrequency of said static inverter.

2. An apparatus as claimed in claim 1 including means for coupling asignal from said static inverter and a signal from said alternate powercomprising first and second phase shift networks for said staticinverter and said alternate power source signals respectively.

3. An apparatus as claimed in claim 2 wherein the means for coupling asignal from said static inverter and a signal from said alternate powersource comprises first and second transformers.

4. An apparatus as claimed in claim 1 wherein the phase detector meanscomprises first and second operational amplifiers.

5. An apparatus as claimed in claim 3 wherein the phase detector meanscomprises first and second operational amplifiers.

6. An apparatus as claimed in claim 1 wherein the means for samplingsaid correction voltage comprises a transistor.

7. An apparatus as claimed in claim 5 wherein the means for samplingsaid correction voltage comprises a transistor.

8. An apparatus as claimed in claim 1 wherein the means for integratingsaid sample correction voltage comprises a resistor and a capacitor.

9. An apparatus as claimed in claim 7 wherein the means for integratingsaid sample correction voltage comprises a resistor and a capacitor.

1. An apparatus for synchronizing the output frequency of a staticinverter with the output frequency of an alternate power sourcecomprising phase detector means for deriving a correction voltagecorresponding to the phase difference between said static inverteroutput and said alternate power source signal output, means for couplinga signal from said static inverter output and a signal from saidalternate power source output to said phase detector means, means forsampling said correction voltage to produce a correction voltage signal,means for coupling said integrated sample correction voltage to avoltage tunable master clock and means for coupling said voltage tunablemaster clock to said static inverter for controlling the outputfrequency of said static inverter.
 2. An apparatus as claimed in claim 1including means for coupling a signal from said static inverter and asignal from said alternate power comprising first and second phase shiftnetworks for said static inverter and said alternate power sourcesignals respectively.
 3. An apparatus as claimed in claim 2 wherein themeans for coupling a signal from said static inverter and a signal fromsaid alternate power source comprises first and second transformers. 4.An apparatus as claimed in claim 1 wherein the phase detector meanscomprises first and second operational amplifiers.
 5. An apparatus asclaimed in claim 3 wherein the phase detector means comprises first andsecond operational amplifiers.
 6. An apparatus as claimed in claim 1wherein the means for sampling said correction voltage comprises atransistor.
 7. An apparatus as claimed in claim 5 wherein the means forsampling said correction voltage comprises a transistor.
 8. An apparatusas claimed in claim 1 wherein the means for integrating said samplecorrection voltage comprises a resistor and a capacitor.
 9. An apparatusas claimed in claim 7 wherein the means for integrating said samplecorrectioN voltage comprises a resistor and a capacitor.